The skin safeguards the human body by acting as the primary defense against environmental stressors. Chronic exposure to environmental stressors can impair the skin barrier, triggering inflammatory responses that disrupt skin homeostasis [1, 2]. Symptoms of skin inflammation, initially acute, can progress to chronic states, contributing to the development of dermatological conditions, including rosacea, psoriasis, and atopic dermatitis [3, 4]. Activation of epidermal keratinocytes and immune cells in response to external stimuli plays a central role in mediating inflammation through elevating intracellular ROS levels, promoting the release of inflammatory mediators and signaling pathways, and ultimately impairing the skin barrier. Stimulation of mitogen-activated protein kinase (MAPK) and nuclear factor kappa B (NF-$\kappa$B)-mediated signaling cascades upregulates pro-inflammatory cytokine and chemokine expression, thereby exacerbating skin inflammation and further compromising dermal hydration and intercellular tight junction integrity [5].

Accordingly, the investigation of natural compounds for treating skin inflammation has attracted increasing research attention. Previous studies have shown that anti-inflammatory plant extracts may serve as promising treatments for cutaneous inflammation by regulating inflammatory mediator expression and reducing cellular damage [6, 7]. Curcuma longa L. (turmeric), a rhizomatous perennial herb, is widely distributed across tropical and subtropical regions, with particular prevalence in Asia [8]. Turmeric has been used as a medicinal herb since ancient times to mitigate and manage a wide range of conditions, including common colds, wounds, skin sores, diarrhea, throat infections, urinary tract infections, menstrual problems, digestive disorders, and liver diseases [9]. It has been extensively used for centuries in Ayurveda (India), Unani (Pakistan), and traditional medicine in Bangladesh for its diverse therapeutic properties, with records of use in India dating back at least 2500 years [10]. Accordingly, research has confirmed the pharmacological relevance of turmeric by identifying numerous bioactive phytochemicals, including curcumin, terpenes, phytosterols, flavonoids, and other phenolic compounds, in various plant parts, which exhibit anti-allergic, anti-atopic dermatitis, antioxidant, antibacterial, and anti-tumor activities [8, 11]. Although considerable progress has been achieved, most investigations have concentrated on the turmeric rhizome. Data on the chemical composition, bioactivities, and therapeutic relevance of the aerial parts, particularly leaves and pseudostems remain limited. Their potential roles in modulating skin inflammation, redox homeostasis, and immune regulation have not been fully elucidated, emphasizing a critical knowledge gap that the present study seeks to address.

In line with this, our collaborative study identified twenty-one phenolic compounds in turmeric leaf and pseudostem extract (CLE) [12]. Furthermore, we previously examined the pharmacological effects of CLE against allergic responses and atopic dermatitis using both cell-based and organismal models [8, 13]. Building on previous studies, this work focuses on exploring the effects of CLE on inflammatory responses in the skin using both cellular and animal models. Thus, the current study investigates the inflammation-regulating properties of CLE in vitro, using tumor necrosis factor (TNF)-$\alpha$/interferon (IFN)-$\gamma$-treated HaCaT keratinocytes (HK), and in vivo, through a 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced ear edema mouse model.

The identification of plant-derived anti-inflammatory treatments for skin inflammation is essential, given the growing demand for safer and more sustainable alternatives to synthetic drugs. Conventional treatments, such as corticosteroids, are effective but can cause adverse effects with prolonged or inappropriate use. These effects include skin atrophy, telangiectasias, and other local or systemic complications, highlighting the need for bioactive compounds with minimal toxicity [17]. Natural extracts, particularly those rich in polyphenols and flavonoids, have demonstrated both antioxidant capacity and anti-inflammatory properties [18, 19]. According to a collaborating research group, four major compounds, vanillic acid, p-coumaric acid, 4-methylcatechol, and afzelin, were quantified among the 21 phenolic compounds identified in CLE by LC-ESI-MS analysis. Their quantified contents were 0.431 $\pm$ 0.002 mg/g for p-coumaric acid, 0.199 $\pm$ 0.001 mg/g for 4-methylcatechol, 0.129 $\pm$ 0.000 mg/g for vanillic acid, and 0.074 $\pm$ 0.000 mg/g for afzelin [12]. Earlier research has extensively reported the anti-inflammatory effects of these phenolic compounds [20, 21, 22, 23]. For instance, a study demonstrated that vanillic acid possesses anti-inflammatory activity in lipopolysaccharide-stimulated mouse peritoneal macrophages through the inhibition of NF-$\kappa$B signaling, stabilization of I$\kappa$B$\alpha$, and reduction of caspase-1 activity [24]. p-Coumaric acid has been reported to exhibit notable anti-inflammatory activity in adjuvant-induced arthritic rats by decreasing TNF-$\alpha$ levels and circulating immune complexes, supporting its potential role in the overall activity of CLE [25]. Furthermore, 4-methylcatechol has demonstrated anti-inflammatory activity in a rheumatoid arthritis model by suppressing pro-inflammatory cytokines, inhibiting M1 macrophage polarization, and modulating the NF-$\kappa$B pathway [22]. Afzelin effectively reduces the proinflammatory effects induced by particulate matter in human keratinocytes [23]. Findings of our study demonstrate the inflammation-modulating effects of CLE may arise from the synergistic or additive actions of these bioactive compounds. This potential highlights their promise as candidates for skin-protective therapies.

Aging and skin inflammation are strongly influenced by the accumulation of cellular ROS. Our analysis revealed that stimulated HK displayed markedly elevated ROS accumulation. Nevertheless, CLE dose-dependently suppressed intracellular ROS accumulation. A comparable reduction in ROS levels has also been reported with pure compounds in cytokine-stimulated HK [15]. This finding suggests a promising avenue for further investigation into their potential anti-inflammatory properties. TSLP, IL-25, and IL-33 are key cytokines that regulate immune responses in inflamed skin. These molecules activate both innate and adaptive immunity, playing a crucial role in promoting inflammation and tissue remodeling [15]. The outcomes of the study demonstrated that CLE markedly downregulated the mRNA expression levels of above cytokines in in vitro RT-qPCR. These findings align with earlier reports describing the skin’s anti-inflammatory mechanisms [26]. Furthermore, IL-6 and IL-8 facilitate the immune cells recruitment to sites of inflammation [27, 28]. TNF-$\alpha$ and IFN-$\gamma$ strengthen inflammatory cascades, while IL-1$\beta$ exacerbates tissue damage by activating pro-inflammatory signaling. Moreover, RANTES and TARC contribute to chronic inflammation by promoting T-cell recruitment [29]. CLE noticeably reduced the mRNA expression of chemokines, epithelial cytokines, and pro-inflammatory mediators. The observations correspond with previous reports demonstrating the anti-inflammatory effects of plant-derived extracts [30].

The NF-$\kappa$B/MAPK pathways drive inflammatory mediator expression, and their inhibition offers a promising strategy to mitigate skin inflammation and tissue damage [5, 16]. The investigation revealed that CLE treatment markedly inhibited NF-$\kappa$B p65 phosphorylation and its nuclear translocation stimulated HK. Additionally, suppression of MAPK phosphorylation confirmed that CLE inhibits NF-$\kappa$B/MAPK signaling involved in inflammation. A previous review highlighted turmeric and its active compounds as modulators of these pathways in inflammatory disorders and pain, thereby supporting the relevance of the present findings [31].

Similarly, preserving skin barrier integrity is crucial for controlling inflammation, as it depends on HA production, the regulation of skin moisturization-related proteins, and the activity of tight junction proteins, factors that collectively support hydration, barrier stability, and immune defense [32]. HA promotes hydration and tissue repair, while moisturization-regulating proteins sustain epidermal homeostasis. Tight junction proteins further strengthen barrier function, limiting antigen penetration and immune hyperactivation [5, 33]. Nevertheless, pro-inflammatory cytokines, primarily controlled through NF-$\kappa$B and MAPK signaling pathways, disrupt these protective mechanisms and exacerbate skin damage [34]. In this study, TNF-$\alpha$/IFN-$\gamma$ stimulation of HK resulted in decreased expression of involucrin, filaggrin, and LEKTI, key proteins involved in epidermal barrier formation, cell adhesion, and skin desquamation regulation. However, CLE pretreatment enhanced LEKTI, involucrin, and filaggrin expression, while suppressing KLK5, PAR-2, and PLA-2, thereby supporting its role in promoting skin hydration. These findings suggest that CLE protects against TNF-$\alpha$/IFN-$\gamma$-induced moisture loss, consistent with outcomes reported in a previous study [5]. Furthermore, administration of CLE replenished HA levels and upregulated tight junction proteins such as occludin, ZO-1, and multiple claudins (claudin-1, -4, -7, and -23), thereby reinforcing its role in preserving skin barrier function during inflammation.

In in vivo studies, epithelial- and epidermal-derived innate cytokines are upregulated during innate immune responses, subsequently activating Th2-type adaptive immunity. This outcome enhances the release of type 2 cytokines, which in turn trigger skin inflammatory processes [35]. Notably, TSLP overexpression has been shown to drive type 2 inflammatory responses in mouse models, highlighting its pivotal role in the pathogenesis of skin inflammatory disorders [36]. This study highlights how TPA exposure provokes inflammation by stimulating crucial signaling cascades, resulting in elevated secretion of cytokines and chemokines. Such molecular events enhance immune cell recruitment and inflammatory activity, ultimately contributing to observable histopathological alterations [15]. The observed dose-dependent reduction in ear thickness, redness, and swelling can be attributed to the ability of CLE to modulate inflammatory mediators and suppress histopathological changes, including epidermal hyperplasia, hyperkeratosis, epidermal thickness, inflammatory cell infiltration, and tissue damage. Molecular analyses of ear tissues using RT-qPCR and western blotting revealed inflammatory patterns in ear swelling in edema mice, which were comparable to those examined in the PC group. Furthermore, CLE treatment markedly reduced COX-2 and iNOS levels in the ear tissues, highlighting its anti-inflammatory potential. Taken together, these findings suggest that CLE possesses considerable pharmacological potential as a natural anti-inflammatory agent for the skin.

Although our data demonstrate a clear local reduction of inflammation by topical CLE in the TPA-induced ear edema model, further studies using systemic inflammation models and systemic administration of CLE are required to determine whether CLE also exerts systemic anti-inflammatory effects. These experiments are beyond the scope of the present study and are planned for future work. Moreover, while the current results are encouraging, certain limitations should be acknowledged. For the further development of CLE as a therapeutic candidate, future research should address long-term safety, investigate systemic and chronic effects in diverse animal models, and conduct clinical testing to confirm efficacy and safety in humans.

Overall, the data suggest that CLE has significant bioactive potential as a natural anti-inflammatory agent for the skin. In vivo, CLE significantly alleviated inflammatory symptoms and modulated molecular mechanisms in TPA-induced ear edema, demonstrating its efficacy in a skin inflammation model. These outcomes collectively highlighted the pharmacological usefulness of CLE as a natural, plant-derived anti-inflammatory agent, offering a promising alternative for managing skin inflammation.

The datasets used and analyzed during the current study are available from the corresponding author on reasonable request.

AMKJ, and KGISK performed the research. KJ, JL, SL, and GA provided help and advice on the experiments. AMKJ, KGISK, KJ, JL, HMCBD, HMKR, LW, and JSK performed data analysis, curation, and visualization. SL, and GA provided funding and resources. SL, and GA were responsible for overall project supervision. AMKJ wrote the manuscript, and KGISK, SL, and GA contributed to its review and editing. All authors contributed to editorial changes in the manuscript. All authors read and approved the final manuscript. All authors have participated sufficiently in the work and agreed to be accountable for all aspects of the work.

All in vivo experiments were conducted following the guidelines of the Ethical Committee of Chonnam National University, South Korea (permission number CNU IACUC-YS-2022-7), and followed the recommendations of the American Veterinary Medical Association (AVMA).

This work was supported by the Korea Institute of Planning and Evaluation for Technology in Food, Agriculture and Forestry (IPET) through the Technology Commercialization Support Program, funded by the Ministry of Agriculture, Food and Rural Affairs (MAFRA) (122052032HD030).

The authors declare no conflict of interest. We confirmed that there is no conflict of interest between this study including its experimental conception and design, data collection, acquisition, analysis, and interpretation, and the institution “French Korea Aromatics Co., Ltd.”. Given his role as the Guest Editor, Lei Wang had no involvement in the peer review of this article and has no access to information regarding its peer review. Full responsibility for the editorial process for this article was delegated to Catarina Rosado.

Supplementary material associated with this article can be found, in the online version, at https://doi.org/10.31083/FBL42888.